TSB has been fined nearly £49m for an IT meltdown in 2018 that caused chaos and left its customers unable to access online accounts for several weeks.
The UK's financial regulator says the failings were widespread and serious and led to significant disruption.
All areas of TSB's services were affected, including branch, telephone and online banking.
The Financial Conduct Authority says the problems were not fully resolved for eight months and a significant proportion of its 5.2 million customers were affected by the initial issues.